This invention relates to a group radio communication system which implements point-to-multipoint communications. More specifically, the present invention relates to a point-to-multipoint radio sub-network which is formed using a cellular radio infrastructure wherein subscriber radios within the sub-network control the selection of channels over which point-to-multipoint communications take place.
Point-to-multipoint (PTM) refers to a communication circuit in which a single signal goes from one originating group member to many destination or target group members. PTM communication can be implemented by sharing common communication resources among many users. PTM communication has been long practiced in connection with commercial broadcast radio and television, where the origination point remains static and the communication resources are allocated for very long durations. However, the origination point may also shift, as occurs in two-way and dispatch radio.
A PTM communication session may take place for an indefinite period of time on the scale of weeks, months, or years, for several hours, or for a shorter duration. Within a PTM communication session, a monolog occurs when one group member is originating information that is being broadcast to the other members of the group. The duration of a monolog is desirably controlled by the group member originating the monolog. When the originator ceases to originate information, the monolog ceases. Desirably, that group member or other group members may originate another monolog thereafter within the same communication session; however, nothing requires any group member to originate a monolog at any given instant. For voice communications, a monolog typically lasts only a few seconds, although nothing requires any particular duration.
In contrast to the long-term resource allocation of PTM, point-to-point (PTP) communication refers to a temporary circuit dedicated to the communication and with essentially two ends. A PTP communication is often referred to as a xe2x80x9ccall.xe2x80x9d A call setup process is performed to allocate the resources which will be dedicated to the call. Upon completion of the call, the resources used to transport communications are typically de-allocated whereupon they may be re-allocated to a different call. In voice communications, a call may last for any duration, but a typical call lasts for only a few minutes. During a typical voice call, the identity of the talking party shifts between the ends of the circuit many times during the call, with each party typically talking for only a few seconds at time.
A conference call represents a hybrid between PTM and PTP. A conference call is typically implemented by forming one PTP circuit for each end of the conference call and bridging the other ends of each PTP circuit together. Many users share the pool of PTP circuits, but the PTP circuits are allocated during a call setup process and de-allocated after the call.
Potential advantages of PTM include a more efficient use of connectivity resources, less expense due to the more efficient use of resources, and ease of providing group connectivity. PTP has advantages of privacy and better odds of being able to provide connectivity between two given ends, assuming a large pool of resources for allocation to calls. Accordingly, PTP connectivity has become popular, and a large infrastructure of wire line, radio, and fiber resources has been built to provide PTP connectivity. However, a need exists for PTM connectivity, particularly in connection with mobile radio communications, which tend to be more expensive due to the scarcity of radio spectrum resources, and in connection with groups, such as business, civic, and military organizations.
Conventional PTM or group radio communication systems suffer several problems which limit their ability to capitalize on the advantages potentially achievable over PTP systems. One problem is that conventional group radio systems fail to use the RF spectrum available to the system efficiently. This failure results in undesirably high connectivity costs and defeats one of the advantages that PTM potentially has over PTP. In addition, it limits the number of subscribers that can be connected together in a group.
For example, if a conventional group radio system follows a conference call paradigm and uses different channels for different users of a group, with the different channels being bridged together, at least as many of the scarce radio channel resources are used for the entire PTM session as would be used to implement the same number of PTP calls for that time period. The number of participants in a group of subscribers will be limited to the number of channels available in a given area. Even if a conference call paradigm is not followed and multiple subscriber radios share some common RF channels to convey user traffic, RF spectrum inefficiencies nevertheless can result if additional RF channels are required to carry signaling and to conduct signal acquisition.
Another problem is that conventional group radio systems often fail to use existing communication infrastructures efficiently, resulting in increased costs and limited coverage areas. Existing communication infrastructures, and particularly cellular radio infrastructures, are typically configured to optimize the delivery of PTP communications. However, infrastructure costs are typically low on a per-user basis because they are shared by a vast number of users, and the coverage area may be up to worldwide.
Another problem is that conventional radio systems often adopt practices that, when applied to a group radio system, fail to provide rapid session management response times. If a group radio system were to follow a PTP call paradigm and engage in a call setup process for each monolog, in which channels are allocated on a monolog-by-monolog basis, an excessive amount of latency would exist between the time a group member wishes to initiate a monolog and when the channels are actually allocated so that the monolog may commence. While subscribers may tolerate lengthy latencies for a PTP call setup, lengthy latencies associated with each PTM monolog would be extremely dissatisfying for subscribers. This problem would be exacerbated if a satellite-based cellular radio infrastructure were relied upon in some way to implement the group radio system because latencies inherent in propagation delays associated with satellite communications would be added to channel allocation delays.
Another problem is that conventional radio systems often adopt practices that, when applied to a group radio system, cause battery-powered devices to consume excessive power. Battery powered devices, such as mobile radios and satellites, should consume as little power as possible while still accomplishing their tasks so that battery reserves are maximized and/or smaller batteries may be used. When such devices are incorporated in group radio communication systems, transmissions from such devices which are not necessary to convey subscriber traffic lead to excessive power consumption. However, conventional radio systems often adopt system designs that cause excessive transmissions for system overhead purposes, such as signaling, maintaining traffic channels, and managing channel selection, rather than for the conveyance of subscriber traffic.
Yet another problem is that conventional radio systems can adopt practices that, when applied to a group radio system, lead to unreliable operation. Fading channels and dropped calls are well known vexations of radio communications systems. Conventional radio systems adopt system designs for which the goal, which is met with varying degrees of success, is to have radios operate on the highest quality channels available at each instant. However, the problem faced by PTM radio systems is more acute than that faced by PTP systems. If a channel fades or is dropped for the monolog originator, then all members of the group, not just the couple of participants in a PTP call, are affected. Hence the consequences of a dropped call are worse in a PTM radio system.
Moreover, a group member in a PTM communication session, which may span a considerable duration, should listen to a group broadcast continuously throughout the session whether or not a monolog is active because a monolog may start at any time. Conventional radio system design practices in which radios spend considerable time scanning for better channels are unworkable in a group radio communication system because they would reduce the time available for listening to the group broadcast. Of course, a group radio might employ two receivers so that one receiver could continuously scan for better channels while the other listens to the group broadcast, but this approach would increase cost and power consumption, thus lessening the potential advantages of a PTM radio system.